Method of operating a vapor generating, superheating and reheating unit



INVENTOR HELLER L. W. HELLER AND REHEATING UNIT 2 Sheets-Sheet 1 /ECONOMIZER L BY 'A'TToRNEY May 5, 1959 I METHOD OF OPERATING A VAPOR GENERATING, SUPERHEATING Filed March 25, 1955 May 5, 1959 LBW HELLER 2,884,910

METHOD OF OPERATING VAPOR GENERATING, SUPERHEATING AND REHEATING UNIT 7 Filed March 25, 1955 2 Sheets-Sheet 2 wzti tffixw'ea f-1a2 I r /160 a 0 244\- ffITlTTi i i E ELLER M'TORNEY United States Patent METHOD OF OPERATING A VAPOR GENERAT- ING, SUPERHEATING .AND REI-IEATING UNIT 1 Lewis W. Heller, Yardley, Pa., assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Application March 25, 1955, Serial No. 496,765

7 Claims. (Cl. 122-479) ing a separately fired vapor reheater furnace, and a separately fired vapor superheater furnace, with the walls of the furnaces including vapor generating tubes normally receiving radiantly the transmitted heat from high temperature heating gases. In connection with these separately fired furnaces, the unit involves convection vapor heating sections including a vapor superheater preferably subject only to the gases of the superheater furnace, and a vapor reheater subject to the gases of the reheater furnace. Combined with these elements the invention involves reheat vapor temperature control means, includ ing recirculated gas means introducing into the reheater fuinace heating gases which have been cooled by passage over at least a part of the convection sections. Included also, in the combination of the invention, is superheater vapor temperature control means including a recirculated gas system introducing into the superheater furnace similarly cooled heating gases.

The invention also involves a method of controlling reheat and superheat temperatures effected by the vapor generating, superheating and reheating unit. The method involves the control of reheat temperature by introducing withdrawn and partially cooled gases from a zone beyond the vapor heating Zones into the reheater furnace, and increasing the flow of the cooled gases so introduced,

as the rate of firing of the reheater furnace and the rate of vapor generation is decreased as a result of decreased vapor demand. These acts may be referred to as reheat vapor temperature control means involving the introduction of recirculated gases in such a manner that these gases have a substantial effect by way of reducing rates of heat absorption by the vapor generating tubes, and increasing the rate of heat absorption in the reheater. Invention of the method also involves the introduction of similarly cooled heating gases, as tempering gases, into the superheater furnace, and the increasing of such flow of tempering gases as the rate of vapor generation and rate of furnace firing increase. This method permits the maintaining of superheat temperature values of the order of 1100" F. by the optimum combustion of a slag forming fuel, said combustion being carried out at high temperatures above the fusion temperature of the ash in the fuel. The method also permits the effecting of such combustion in a relatively small and inexpensive furnace, and with adequate control of slagging conditions at the position of the superheating zone in order that the availability of the unit shall be of a high value.

2,884,910 Patented May 5, 1959 convection heating surfaces, and the rate of introduction of such tempering gases is a maximum at top load and a minimum of low load at the bottom of a Wide load range. Thus, with the recirculated gases being introduced to the reheater heating gas zone in a manner related to the changing load, which is the reverse of the manner of introduction of the tempering gases to the superheater heating gas zone, the two systems of returning gases to the heating gas zones may be so combined that one fan is employed and operated in such a manner that the load on the fan does not vary over a wide range.

In a more specific sense the invention involves such a vapor generating, superheating and reheating unit as that above described, the unit being further characterized as having a rectangular setting including upright walls defined by upright steam generating tubes. Some of the steam generating tubes constitute a division wall which is common to the secondary furnace chamber of the superheater heating gas zone and to the secondary furnace chamber of the reheater heating gas zone. The first of these zones is supplied with high temperature heating gases by one or more cyclone furnaces burning a particle form slag forming fuel at high temperatures above the fusion temperatures of the slag in the fuel, and in such a manner that combustion is substantially complete within the cyclone. Similarly, the reheater heating gas zone and its secondary furnace chamber is supplied with high temperature gases by one or more cyclone furnaces operating in a similar manner, preferably the first set of cyclones for the superheater furnace or its gas zone directs its gases toward a division Wall common to both secondary furnace chambers, and the cyclone or cyclones for the reheater furnace or reheater heating gas zone directs its gases in an opposite direction, and toward the division wall.

In another sense the invention involves an arrangement such as that above described, providing a method of operation wherein the high pressure steam superheater, and the low pressure steam reheater are disposed in parallel gas passes, and a recycled gas system, or systems, is utilized in connection with the furnaces, in different manners, so as to develop different effects. In this arrangement the high pressure superheater furnace, or the superheater heating gas zone will inherently provide gases of maximum temperature at maximum unit output, and the recycled gases will be introduced and mixed with the unrecycled gases as a tempering medium to control slagging conditions and to control vapor superheat temperature. In the other furnace serving the reheater, the area of the vapor generating heat absorbing surfaces and their arrangement will be such as to provide the optimum furnace exit gas temperature to give optimum reheat temperature at maximum output. As the unit output drops off, the recirculated gases are introduced into this furnace in a manner to reduce the wall tube heat absorption and to provide increased heat content in the gases leaving the furnace to obtain optimum reheat. The percentage of recirculated gases passing through the reheater furnace is increased as the steam output of the unit is decreased, while the percentage of recycled gases introduced into the upper portion of the superheater furnace is lowered as the output is decreased. In connection with this embodiment of the invention, itis noted that, where the recirculated gases are introduced into the reheater gas zone to afiect v. the water Wall heat absorption, it does not have a similar effect on the water walls of the furnace chamber serving the high pressure steam superheater.

In another and more specific aspect of the invention, involving the setting indicated above, the superheater and the reheater are in a series with respect to the fiow of heating gases generated in the superheater furnace. The recycled gases are introduced into the high temperature superheater furnace as tempering gases, while the recycled gases are introduced into the furnace from which. the newly developed combustion products flow to the reheater as recirculated gases to modify furnace wall heat absorption, and thereby raise reheat with the reduction of steam output. In this aspect of the invention, the high pressure steam superheater has been split into primary and secondary sections with only the latter section served by the gases of the superheater furnace.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification, but for a better understanding of the invention, its operating advantages and the specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described preferred embodiments of the invention.

In the drawings:

Fig. 1 is a diagrammatic side sectional view of an embodiment of the invention wherein the secondary superheater and the reheater are in series with respect to gas flow from one of the furnaces; and

Fig. 2 is a diagrammatic view in the nature of a side sectional elevation of another embodiment of the invention in which the reheater is entirely located within one of two parallel gas passes leading from separately fired furnace chambers.

The steam generating, superheating and reheating unit illustrated in Fig. l of the drawings preferably involves a vertically elongated setting of rectangular cross-section having front and rear walls 10 and 12 along which there are arranged upright vapor generating tubes 14 and 16, having their lower ends connected to the drum or header 18, and their upper ends connected to a steam and water drum 20, in a manner well known in the art.

The side walls of the setting are similar to the front and rear walls 12 and 14, and they also include vapor generating wall tubes which are connected by appropriate headers and connections to the lower drum 18, and the steam and water drum 20. Other vapor generating wall tubes 22 are arranged in wall forming alignment to constitute a division wall separating two parts of the setting,

including the secondary furnace chamber 24 of the reheater heating gas zone, and the secondary furnace chamber 26 of the superheater heating gas zone. Intermediate parts of the division wall tubes 22 are disposed along the upper and lower sides 28 and 30 respectively of the arch intermediate the height of the unit. The upper parts 32 of some of the division wall tubes 22 form a screen extending across the path of gases leading to the convection section gas pass at the top of the unit and others of the division wall tubes are bent to the right along the inclined wall 34 at the bottom of the gas pass, and then upwardly in screen formation at 36. Upwardly beyond this screen these tubes continue to connections with the steam and water drum, as shown. The lower parts of some of the wall tubes 14 extend along the left hand wall of the primary furnace chamber 42, past the throat 44 of the cyclone 46 and then along the floor 48 of the primary furnace chamber to the drum 18. Others of the tubes 14 are bent to the right outwardly of their wall alignment to define the wall portions 50 and 52 of the primary furnace chamber 42, and then the screen of spaced tubes 54 extending across the upward flow of gas pass from the bottom of the primary furnace chamber. From this screen the tubes extend downwardly as indicated at 56 4 to the header or drum 18. In the zone of the primary furnace chamber it is to be understood that the wall tubes are associated with inter-tube closures, such as those involving high temperature refractory material and metallic inter-tube studs secured to the tubes.

Similarly, and at the opposite side of the setting some of the wall tubes 16 have their lower parts extending past the throat 60, and the front wall 62 of the cyclone 64, and thence downwardly along the wall 66 and the floor 68 of the primary furnace chamber 70, to the drum 1%.

Others of the wall tubes 16 have their lower parts bent to the left at an elevation just above the elevation of the cyclone 64, so as to form the downwardly in clined screen tubes 72 extending across the upward flow of gases from the primary furnace chamber 70.

The heating gases flowing through the superheater furnace originate in the cyclone furnace 64 which is preferably of the type indicated in the U.S. patent to Kerr et al. 2,594,312, of April 27, 1952. The gases pass through the throat 60 of the cyclone into the lower, or primary furnace chamber 70, through the upper, or secondary, chamber 26 of the superheater furnace, and thence over the horizontally spaced pendent tubes of the secondary superheater 80. Thence the gases pass through the spaces between the screen tubes 22 and into the upper part of the secondary furnace chamber 24 for the reheater furnace. Within this chamber the gases from the superheater furnace join the gases in the reheater furnace which have originated in the cyclone furnace 46. The combined gases pass from the top of the secondary furnace chamber 24 across the horizontally spaced tubes of the pendent reheater 82. The gases next pass over the tubes of the pendent primary superheater 84; then between the screen tubes 36; and thence over the horizontally spaced upright tubes of the convection econo mizer 86.

The primary superheater 84 has an inlet header 88 receiving saturated steam through the line 90, from the steam and water drum 20. Steam flows from the primary superheater outlet header 92 through a line 94 (including an attemperator, not shown) to the inlet header 96 of the secondary superheater 80. From the outlet header 98 of the secondary superheater the superheated steam flows through a line 100 to the inlet of a high pressure turbine stage 102. From the exhaust of this turbine stage the steam flows through a line 104 to the inlet header 106 of the reheater 82. From the outlet header 108 of the reheater the reheated steam flows through a line 110 to the inlet of a low pressure turbine stage 112.

For so coordinating and proportioning superheater and reheater absorbed heat and heat absorbed in vapor generation that superheated vapor temperature and reheated vapor temperature may be maintained at predetermined values over a wide range of rates of vapor generation, the invention employs a superheat control method which includes the introduction of recycled lower temperature gases as tempering gases to the upper furnace chamber of the superheater furnace and the introduction of such gases as recirculated gases, to the primary furnace chamber 42 of the reheater furnace. The flow of the recycled gases to these different furnaces is regulated from changes in vapor generating load, and from the changes in superheated steam temperature or reheated steam temperature to maintain the steam temperatures close to a predetermined value, or values. For example, as the vapor generating load decreases, a decreasing percentage of the recycled gases is introduced as tempering gases into the upper part of the superheater furnace chamber, and an increasing percentage of recirculated gases is introduced into the primary furnace chamber 42 of the reheater furnace. For effecting such flow of recycled gases the Fig. 1 unit shows a recycled gas system, including a fan which has an inlet communicating with a branch gas pass 122 leading from the main flue 124 so as to withdraw part of the heating gases cooled by passage over the convection heating surfaces of the unit. The outlet 126 of the fan communicates with a duct 128 leading to a windbox 130 for the recirculated gases. This windbox has a number of outlet passages between successive spaced tubes 40, whereby the recycled gases pass from the windbox into the upper part of the primary furnace chamber 42. The duct 128 also has a tempering gas branch 132 leading to a transverse windboX 134 communicating with the secondary furnace chamber 26 through outlets formed between successive tubes 16.

The Fig. 2 unit includes a reheater furnace, the main components of which are a cyclone furnace 150, a primary furnace chamber 152, and a secondary furnace chamber 154. The heating gases from this reheater furnace pass upwardly over the parallel flow section 156 of the reheater, and then over the counter-flow section 158 of the reheater. Beyond the reheater the gases pass over the relatively small economizer section 160. These convection heating sections, as well as the secondary furnace chamber 154, are disposed in a separate gas pass, the walls of which include upright vapor generating tubes, such as 162 and 164. The latter tubes constitute part of a division wall separating the above indicated gas pass from a similar parallel gas pass 184 for the heating gases from the superheater furnace. This furnace has as its main components the cyclone 170, the primary furnace chamber 172, and the secondary furnace chamber 174. The heating gases from the superheater furnace pass upwardly over the parallel flow section 176 of the secondary superheater, and then over the counter-flow section 178 of that superheater. Thence the gases pass upwardly over the bank of tubes forming the primary superheater 180, and beyond this primary superheater the gases pass over the large economizer section 182.

The gas pressures in the reheater furnace and in the superheater furnace, and in their associated gas passes 184 and 186 may be controlled by the gas flow regulators at the outlets of the gas passes. To this end the drawing shows the dampers or gas flow regulators 190 disposed at the outlet of the gas pass 184, and the dampers or gas flow regulators 192 disposed at the outlet of the gas pass 186.

The vapor generating tubes of the Fig. 2 unit are appropriately connected into a fluid circulation system which may include a steam and water drum similar to that shown in Fig. 1, at the upper part of the unit. The upper ends of the vapor generating tubes are connected to this drum and their lower ends are connected to lower headers or drums, such as those shown at 200, 202 and 204, some of the vapor generating wall tubes 162 being connected to the header 204 and others bent to form the inclined wall 206, and the upright wall 208, along with the screen 210 of the primary furnace cham her 152.

The vapor generating wall tubes 207 at the left hand side of the gas pass 184 similarly connect with the header 200 and the header or drum 202, parts of some of these tubes forming the wall sections 208 and 210, and a screen 212 of the primary furnace chamber 172. At the position A alternate wall tubes 207 are bent out of their wall alignment to form ports through which recycled tempering gases flow into the secondary furnace chamber of the superheater furnace. The division wall vapor generating tubes 164 are similarly arranged at the position B to form ports between some of these tubes for the entry of recycled tempering gases into the superheater furnace secondary furnace chamber. Below the position B, and at the position C, parts of alternate vapor generating and division wall tubes 164 are bent to the right as indicated at 220, and the remaining tubes are bent to the left, as indicated at 222 to form gas com municating passageways between the secondary furnace chamber for the reheater furnace, and the secondary furnace chamber for the superheater furnace. Below the position C the vapor generating division wall tubes are connected to the drum or header 202.

Like the Fig. 1 unit the vapor generating tubes of the Fig. 2 unit deliver mixtures of steam and water to a steam and water drum similar to that shown at 20 in Fig. 1. In this drum the steam is separated from the water and the separated steam passes to the inlet header 230 of the primary superheater 180. From this header the steam flows through the successive and serially connected return bend sections of the primary superheater 180, to the outlet header 232 of the primary superheater. From this outlet header the steam flows through an attemperator 234, in which the superheated steam may have water injected into it for preventing any substantial rise in superheated steam temperature above a predetermined value, at a small upper part of the entire range of vapor generating load. From the attemperator 234 the steam passes to the tubes of the secondary superheater sections 178 and 176, and then to the outlet header 236. From this outlet header the steam passes through a line 238 to the inlet of a high pressure turbine stage 240. From the outlet 242 of this turbine stage the steam passes through a line 244 to the reheater inlet header 246. From this header the steam passes through the tubes of the reheater sections 156 and 158 to the reheater outlet header 246. From this header the steam passes through a line 248 to the inlet 250 of the low pressure turbine stage 252.

Like the Fig. 1 embodiment the Fig. 2 unit involves a recycled gas system for controlling superheat and reheat temperatures for over at least a major part of the entire load range of vapor generation. Also like the Fig. 1 unit, this recycled gas system employs the recycled gases as tempering gases, introduced into the secondary furnace chamber of the superheater furnace at a position just ahead of the secondary superheater, with the percentage of tempering gases increased as the load increases to maintain the gas temperatures at the gas inlet of the superheater within safe values as regards superheater tube metal, and also as regards the slagging conditions when a cyclone furnace is employed to burn a slag forming fuel at temperatures above the fusion temperatures of the slag. The Fig. 2 unit also has a percentage of the recycled gases introduced as recirculated gases into the primary furnace chamber of the reheater furnace, the percentage of recirculating gases thus flowing into the primary furnace chamber of the reheater furnace increasing as the vapor generating load on the unit decreases.

The recycled gas system of the Fig. 2 unit includes a recycling gas fan 260, having its inlet 262 connected by ductwork 264 to a recycled gas inlet 266, communicating with the gas flow in the gas pass 186 at a position just ahead of the gas flow regulators 192. This ductwork may be also in communication with an inlet 268 communicating with the gas flow in the gas pass 184 at a position just ahead of the gas flow regulators 190. The outlet 270 of the fan 260 is connected with ductwork 272 which has a branch 274 leading to a windbox 276, which has a series of outlets disposed across the upper part of the right hand wall of the primary furnace chamber 152 formed between successive or alternate parts of vapor generating wall tubes 162, the flow of recirculated gases through the ductwork branch 274 being controlled by a damper or gas flow regulator 278 and the flow and distribution of the gases introduced into the furnace chamber 152 may be controlled by separate dampers in the passages between successive tubes 162.

The ductwork 272 leading from the outlet of the recirculating gas fan 260 has another branch 280 leading to the windbox 282, and the windbox 284, for the introduction of tempering gases in the upper part of the secondary furnace chamber of the superheater furnace. The windbox 282 has outlets formed between some of the wall tubes 207 and distributed over the width of the gas pass face for the entry of tempering gases at the position A, and a similar arrangement of the division wall tubes 164 provides a row of recycled gas outlets distributed across the division wall at the position B. The flow of such tempering gases into the upper part of the secondary furnace chamber 174 may be controlled by the automatic or manual operation of the damper 281. Similarly, the tempering gas flow to the secondary furnace chamber 26 of the Fig. 1 unit may be controlled by the manual or automatic operation of the damper 133. Manual or automatic operation of the dampers 129 of the Fig. 1 unit or the damper 278 of the Fig. 2 unit may similarly control recirculated gas flow.

Whereas the invention has been described with reference to the details of specific embodiments thereof, it is understood that such description is by way of example and explanation only and not by way of limitation, the invention is rather to be taken as of a scope commensurate with the scope of the subjoined claims.

What is claimed is:

1. In a method of controlling superheat and reheat vapor temperatures of a high pressure vapor used for power purposes, separately burning a slag forming fuel for a vapor reheating gas zone; separately burning a slag forming fuel at temperatures above the slag fusion temperature for a vapor superheating gas zone; radiant- I ly transmitting heat from the combustion gases of said zones to generate vapor in confined fluid streams bordering said zones; establishing separate flows of heat ing gases from said zones with one of said gas flows constituting predominantly gases from the combustion of the reheating gas zone, and the other gas flow constituting predominantly the gases from the combustion for the superheating gas zone; convectionally transmitting heat for reheating a generated vapor from the gases of one of said gas flows; convectionally transmitting heat from the gases in the other said gas flow for superheating the generated vapor; controllably introducing recycled gases as recirculating gases into the reheating gas zone for controlling reheat temperatures as the vapor generating load changes; increasing the flow of introduced recirculated gases to the reheater gas zone as the vapor demand decreases to maintain reheat vapor temperature at a predetermined value; simultaneously introducing recycled and lower temperature gases from a position beyond the convection superheating zone to the superheater gas zone as tempering gases with the percentage of introduction of the tempering gases increasing from a minimum at low vapor generating rate to a maximum at high vapor generating rate to maintain superheating gas temperatures within allowable limits as to its effects upon superheater metal and upon slagging conditions.

2. A vapor generating, superheating and reheating unit comprising walls defining a vertically elongated reheat furnace chamber having means for burning fuel in its lower end and a heating gas pass opening to its upper end, radiantly heated vapor generating tubes lining a wall of said furnace chamber, a bank of convection heated vapor reheating tubes in said gas pass, walls defining an adjoining vertically elongated superheat furnace chamber having means for independently burning a slagforming fuel in its lower end at furnace chamber temperatures above the slag fusion temperature and a heating gas pass opening to its upper end, radiantly heated vapor generating tubes lining a wall of said superheat furnace chamber, a bank of convection heated vapor superheating tubes in said second gas pass arranged to receive slag-laden heating gases from said superheat furnace chamber, means for withdrawing heating gas from one of said gas passes downstream of the convection heated tube-s therein and, introducing withdrawn heating gas into said reheat furnace chamber at a level remote from the heating gas pass opening thereto and in position to affect radiant heat transmission to a major portion of the vapor generating tubes therein, and means for introducing withdrawn heating gas into said superheat furnace chamber in intimate mixing relation with the slag-laden heating gases at a level above said fuel burning means therein and closely subjacent to said vapor superheating tubes in said second gas pass, said heating gas introducing means introducing a sufficient quantity of heating gas into said superheat furnace chamber to reduce the temperature of the mixed gases therein below the slag fusion temperature prior to the mixed gases contacting with the convection heated vapor superheating surface.

3. A vapor generating, superheating and reheating unit comprising walls defining a vertically elongated reheat furnace chamber having means for burning a fuel in its lower end and a heating gas pass opening to its upper end, radiantly heated vapor generating tubes lining a wall of said furnace chamber, a bank of convection heated vapor reheating tubes in said gas pass, walls defining an adjoining vertically elongated superheat furnace chamber of substantially less vertical extent and volume than said reheat furnace chamber and having means for independently burning a slag-forming fuel in its lower end at furnace chamber temperatures above the slag fusion temperature and a heating gas pass opening to its upper end, radiantly heated vapor generating tubes lining a wall of said superheat furnace chamber, a bank of convection heated vapor superheating tubes in said second gas pass arranged to receive slag-laden heating gases from said superheat furnace chamber, means for withdrawing heating gas from one of said gas passes downstream of the convection heated tubes therein and introducing withdrawn heating gas into said reheat furnace chamber at a level remote from the heating gas pass opening thereto and in position to affect radiant heat transmission to a major portion of the vapor generating tubes therein, and means for introducing withdrawn heating gas into said superheat furnace chamber in intimate mixing relation with the slag-laden heating gases at a level above said fuel burning means therein and closely subjacent to said vapor superheating tubes in said second gas pass, said heating gas introducing means introducing a sufficient quantity of heating gas into said superheat furnace chamber to reduce the temperature of the mixed gases therein below the slag fusion temperature prior to the mixed gases contacting with the convection heated vapor superheating surface.

4. A vapor generating, superheating and reheating unit comprising walls defining a vertically elongated reheat furnace chamber having means for burning a fuel in its lower end and a heating gas pass opening to its upper end, radiantly heated vapor generating tubes lining a wall of said furnace chamber, a bank of convection heated vapor reheating tubes in said gas pass, walls including a tubular division wall defining an adjoining vertically elongated superheat furnace chamber of substantially less vertical extent and volume than said reheat furnace chamber and having means for independently burning a slag-forming fuel in its lower end at furnace chamber temperature above the slag fusion temperature and a heating gas pass opening to its upper end, radiantly heated vapor generating tubes lining a wall of said superheat furnace chamber, a bank of convection heated vapor superheating tubes in said second gas pass arranged to receive slag-laden heating gases from said superheat furnace chamber, means including a single gas recirculating fan for withdrawing heating gas from one of said gas passes downstream of the convection heated tubes therein and ductwork arranged to introduce withdrawn heating gas into said reheat furnace chamber at a level remote from the heating gas pass opening thereto and. in position to affect radiant heat transmission to a major portion of the vapor generating tubes therein, and arranged to introduce withdrawn heating gas into said superheat furnace chamber in intimate relation with the slag-laden heating gases at a level above said fuel burning means therein and closely subjacent to said vapor superheating tubes in said second gas pass, said heating gas introducing means introducing a sufficient quantity of heating gas into said superheat furnace chamber to reduce the temperature of the mixed gases therein below the slag fusion temperature prior to the mixed gases contacting with the convection heated vapor superheating surface.

5. In a vapor generating, superheating and reheating unit, the method of operation over a wide generating load range which comprises burning a fuel, conducting the heating gases generated through an elongated chamber in radiant heat transmission to confined streams of a vaporizable liquid in the boundary surfaces thereof and then in convection heat transfer to confined streams of vapor to be reheated, independently burning a slagforming fuel in a second chamber at fusion temperatures above the slag fusion temperature, conducting the heating gases generated through an elongated chamber While radiating heat to confined streams of vaporizable liquid in the boundary surfaces thereof, then conducting the heating gases in convection heat transfer with confined streams of vapor to be superheated, withdrawing partly cooled heating gases from at least one of said heating gas flow paths downstream of the convection heated surface and introducing withdrawn heating gas into the first chamber at a point remote from the convection heated vapor reheating surface and in position to affect radiant heat transmission to a major portion of the vapor generating surface therein, separately introducing so withdrawn heating gas into the second chamber at a point between the combustion zone therein and the convection heated vapor superheating surface, increasing the amount of heating gas introduced into the first chamber on a decrease in the generating load of the unit to thereby reduce the radiant heat absorption of the vapor generating surface therein and increase the vapor reheating effect, and independently increasing the amount of heating gas introduced into the second chamber on an increase in the generating load of said unit to thereby reduce the temperature of the heating gas below the slag fusion temperature prior to contacting with the convection heated vapor superheating surface.

6. In a vapor generating, superheating and reheating unit, the method of operation over a wide generating load range which comprises burning a slag-forming fuel at combustion temperatures above the slag fusion temperature, conducting the heating gases generated through an elongated chamber in radiant heat transmission to confined streams of a vaporizable liquid in the boundary surfaces thereof and then in convection heat transfer to confined streams of vapor to be reheated, independently burning a slag-forming fuel at fusion temperatures above the slag fusion temperature, conducting the heating gases generatcd through a second chamber of lesser volume than the first chamber while radiating heat to confined streams of vaporizable liquid in the boundary surfaces thereof, then conducting the heating gases in convection heat transfer with confined streams of vapor to be superheated, withdrawing partly cooled heating gases from at least one of said heating gas fiow paths downstream of the convection heated surface and introducing a portion of the withdrawn heating gas into the first chamber at a point remote from the convection heated vapor reheating surface and in radiant heat transmission to a major portion of the vapor generating surface therein, and separately introducing another portion of the heating gas so withdrawn into the second chamber at a point between the combustion zone therein and the convection heated vapor superheating surface, increasing the amount of heating gas introduced into the first chamber on a decrease in the generating load of the unit to thereby reduce the radiant heat absorption of the vapor generating surface therein and increase the vapor reheating effect, and independently increasing the amount of heating gas introduced into the second chamber on an increase in the generating load of said unit to thereby reduce the temperature of the heating gas below the slag fusion temperature prior to contacting with the convection heated vapor superheating surface.

7. The method of operating a vapor generating, superheating and reheating unit over a wide generating load range as claimed in claim 5 in which the heating gases generated in the second chamber after passing in convection heat transfer with confined streams of vapor to be superheated are caused to join the heating gases generated in the first chamber before the gases pass in convection heat transfer to the confined streams of vapor to be reheated, so that all of the heating gases generated pass in convection heat transfer to the confined streams of vapor to be reheated.

References Cited in the file of this patent UNITED STATES PATENTS 2,285,442 Kerr June 9, 1942 2,737,930 Rowand et al Mar. 13, 1956 FOREIGN PATENTS 109,830 Australia Feb. 14, 1940 1,045,900 France July 1, 1953 1,085,964 France Aug. 4, 1954 

